The present invention relates generally to satellite navigation system receivers and more particularly to increasing the reliability of such receivers.
Satellite navigation systems (also referred to as navigation satellites) such as GPS (USA), GLONASS (Russia) and GALILEO (Europe) are typically used to accurately determine the position of users possessing navigation receivers. A navigation receiver receives and processes radio signals transmitted simultaneously by a number of satellites. These radio signals include carrier signals which are modulated by pseudo-random binary codes (PR-codes). The codes are typically inversely modulated by binary information symbols.
Each receiver moves with the user possessing the receiver. The receiver that moves with the user is often called a Rover. In differential navigation (DN), the Rover and a Base station receive the same satellite signals from a navigation satellite. The Rover measures a time delay of when the Rover receives the satellite signals relative to when each navigation satellite transmits the signals. The Rover measures the distance (i.e., pseudorange) between the satellite and the receiver by measuring the time the signal takes to propagate from the satellite to the receiver. The pseudorange is this time offset multiplied by the speed of light.
As described above, the Base station receives the same satellite signals as the Rover. The precise coordinates of the Base station are known and the Base station is generally stationary during measurements. The Base station has a navigation receiver which receives and processes the signals of the satellites to generate measurements. These signal measurements are transmitted to the Rover via a communication channel (e.g., wireless). The Rover uses these measurements received from the Base station, along with its own measurements taken with its own navigation receiver, in order to precisely determine its location. The location determination is improved in the differential navigation mode because the Rover is able to use the Base station measurements in order to compensate for the major part of the strongly correlated errors in the Rover measurements.
For each satellite, the time delay measured by the Rover is measured using scales. There is a comparative coarse scale to measure PR-code delay. A code delay is the offset between a Rover's locally generated code and code received from the satellite. Code delay is also known as PR code phase. Delays can also be measured by the phase of the carrier frequency. Such measurements have a small unambiguity range (the period of carrier frequency), but are typically more accurate.
The Rover measures delays using two loops. A Phase Lock Loop (PLL) is used to track carrier phases. A Delay Lock Loop (DLL) is used to measure code delays.
The Rover additionally includes channels. A channel consists of circuitry necessary to track the signal from a single satellite. Specifically, a single satellite channel may be considered as a basic structural unit to measure code and carrier delays relative to a single code and carrier on one frequency from a satellite. Each channel works with publicly accessible signals, for instance, the GPS C/A signal or GPS L2C or likely the GPS L3 or GLONASS C/A, etc.
Processing of the received signal involves the accumulation of successive multiplication products of this signal (which has passed through input and filtering circuits and a frequency converter). A reference carrier and reference code is generated in the receiver. The reference carrier corresponds to the carrier of the received signal for a given satellite, while the reference code corresponds to the PR-code of the same satellite. Components that are responsible for multiplication and accumulation are called correlators while the corresponding process is called correlation.
There are several ways to design channels for a receiver of the satellite signals. Each channel typically includes three paths. The first path calculates in-phase correlation signal I. This correlation signal can be obtained if a first reference carrier is in-phase with the input signal carrier while the reference code is a copy of PR-code modulating the input signal.
When a phasing error of a reference carrier φ and a time offset of a reference code ε exists, then, ignoring interference (for the sake of simplicity), signal I can be represented as:I=μUmR0(ε)cos φ,
where μ is the binary information symbol,
Um is the amplitude of the received signal, and
R0(ε) is the cross correlation function of the PR-code (after passing the receiver's filter) and reference code which is a locally generated replica of PR-code for the input signal.
The second path calculates quadrature correlation signal Q. This signal can be obtained when the second reference carrier is shifted by π/2 from the first reference carrier. The correlation of the reference carrier signals generates a quadrature component of the input signal. The reference code is the same as the reference code in the first path. Signal Q may be written as follows:Q=μUmR0(ε)sin φOrthogonal signals Q and I are used for PLL phase discriminators.
The third path calculates correlation signal dl used for controlling the DLL. To obtain this signal, the first reference carrier in-phase with the input carrier is used, and the reference code consists of short strobe-pulses that match changes of sign of input PR-code chips. Note that the sign of strobe-pulses matches the sign of a chip which follows the strobe-pulse.
In order for the DLL and the PLL of a receiver's individual channels to lock onto a signal, a delay search system searches for a delay and frequency of the received signal. The delay search system sets an initial delay of the reference code to be as close as possible to the input PR-code delay. If a set error of the initial delay does not exceed the limits of a DLL lock-in range, the DLL stops at a steady balance point. The delay search system (which may include a frequency search system) may also set an initial frequency of the reference carrier with an error that does not exceed a PLL lock-in range.
During operation of the receiver, a loss of tracking in DLLs and PLLs may occur because of short-term signal loss when an object shades the receiving antenna. In this case, the delay search system has to lock onto the signal again and resume tracking once the shading is removed.
PLL and DLL bandwidths are often selected to reduce the amount of dynamic error when measuring the delay. Factors that are considered include the movement of satellites and the receiver, frequency fluctuations of a receiver reference oscillator, variations in delays of wave propagation through the atmosphere, instability of reference signal generators, etc.
As the bandwidth increases, the dynamic errors traditionally decrease. However, it is often not possible to increase the bandwidth beyond a certain bandwidth because errors from additive interference (including those caused by the receiver's intrinsic noise) increase. Furthermore, as the bandwidth increases, the risk of cycle slips and loss of lock increases. This risk is often high for channels where a satellite signal is weak, such as for low satellites or satellites partially shaded by foliage. Hence, there exists a well-known constraint which limits a threshold value of a signal-to-noise ratio (SNR) at which controlling loops can provide robust operation.
U.S. Pat. No. 6,313,789, issued on Nov. 6, 2001, describes a method of resolving the mentioned constraint by tracking common disturbances in a wide bandwidth. In particular, that patent describes using individual PLLs to track the phase of each visible satellite's carrier signal and using a common vector loop to track common carrier phase disturbances for the satellites. There is also an individual DLL in each channel for correcting the clock rate of a reference code by a controlling signal from a corresponding PLL.
In a receiver with common controlling loops, signals from individual. PLLs in each channel are added to predicted and corrected signals, with the sum being used to control frequency of individual, digital, numerically controlled oscillators (NCOs) in each channel. The predicted signals are calculated by predicting the satellite movement. The prediction of satellite movement includes compensating for relevant Doppler frequency shifts for carrier frequencies as well as other factors subject to prediction. The corrected signals are generated at the output of a common vector loop that groups the receiver channels. The input of the common vector loop is an N-dimensional vector having vector components of output signals of PLL discriminators in N-channels. This vector is transformed by a least square method (LSM). As a result of the transformation, signals are provided to dynamic (loop) filters and are further projected onto the direction of each satellite, thereby forming corresponding correcting signals.
The common vector loop and N individual PLLs form a multi-loop control system. Some external influences that affect the carrier phase are common for all of the channels. They include the receiver movements and frequency fluctuations of the reference oscillator. These effects are mainly tracked by the common vector loop. Other external effects are different in different channels. They include portions of atmosphere delays, frequency fluctuations of satellite onboard reference oscillators, and errors of predicted satellite movements. These effects are mainly tracked by individual PLLs.
As common effects have a much larger impact compared with the impact of individual effects, the common vector loop typically has a much wider bandwidth and is less steady relative to individual PLLs. Tracking of code delays also benefits from the common vector loop as corrections from individual PLL signals are applied to individual DLLs.
Using both individual circuits and common PLLs, however, present risks with respect to mutual interaction of individual loops. As a result, interchannel interference may occur, which is interference on one channel from a neighboring channel. Interchannel interference may lead to tracking loss or cycle slips in PLLs of different channels after it has occurred in a PLL of one of the channels. To neutralize the interchannel interference, the signal quality in each channel is continuously estimated relative to the signal-to-noise ratio.
Alternatively, it is observed that the permissible limits of the signal quality in each channel are exceeded by an output signal of a PLL discriminator. When the signal quality falls below the permissible threshold, an alarm signal is generated. If this alarm signal has been generated on time, the corresponding channel does not use the common loops until the signal quality returns to an acceptable level. In particular, channels with temporary shading of the satellite signal do not use the common loops when the alarm signal is generated. Measurements of the shaded channel are not used for coordinate calculations.